1. Field of the Invention
This invention relates to a process for producing relatively high molecular weight copolyesters of 1,4-cyclohexanedicarboxylic acid and a mixture of three glycols by a combination of melt-phase polycondensation followed by solid-phase polycondensation.
2. Description of the Prior Art
Solid-phase polymerization following melt-phase polymerization is well known in the art. Normally such solid-phase polymerization is carried out at temperatures around 200.degree. C.
The advantages of this invention are that polyesters having improved properties such as lower odor and lower color can be produced. If a polyester is prepared to high molecular weight in the melt, the high temperatures needed can cause excessive degradation causing odor and color formation. By use of the solid-phase process of this invention, the high molecular weight copolyesters can be prepared at much lower temperatures significantly reducing the possibility of degradation. The process of this invention is particularly useful since part of the copolyester is a polyalkyleneether glycol. Polyalkyleneether glycols have poor thermal stability and it is therefore, desirable to carry out the polymerization at as low a temperature as possible.
U.S. Pat. No. 3,801,547 discloses a solid phase polycondensation process for preparing polyesters derived from an acid component of at least 70 mole percent terephthalic acid and a glycol component of at least 70 mol percent 1,4-butanediol. The polyesters, as disclosed in this patent are prepared by heating solid partially polymerized copolyester having an inherent viscosity of at least about 0.8 to a temperature of about 140.degree.-220.degree. C. Other patents disclosing solid phase polymerization of polyesters include U.S. Pat. Nos. 3,068,204; 3,075,952; 3,117,950 and 3,254,055. Patents disclosing polyetherester elastomers based on polytetramethylene terephthalate include U.S. Pat. Nos. 3,651,014; 3,763,109; 3,755,146; 3,023,192 and 3,784,520. Polyetheresters containing 1,4-cyclohexanedimethanol and polytetramethylene glycol are disclosed in U.S. Pat. Nos. 3,261,812 and 3,243,413.
The present invention provides a process for producing a relatively high molecular weight copolyester. The method comprises, first, melt-phase copolymerizing to an inherent viscosity of about 0.50 to about 1.10 a dibasic acid component and a glycol component, the dibasic acid component consisting essentially of 1,4-cyclohexanedicarboxylic acid having a transisomer content of at least 70%. The glycol component comprises, based on 100 mol percent glycol, from about 97 to about 70 mol percent 1,4-cyclohexanedimethanol, about 3 to about 30 mol percent of a glycol having 2 to 5 carbon atoms and from about 15 to about 80 weight percent of the total polymer of a polyalkyleneether glycol having a carbon to oxygen atom ratio of from 2 to 1 to 4 to 1 and a molecular weight of from about 400 to about 4000. As the second step, the relatively low inherent viscosity polymer obtained from the first step is solid-phase polymerized in particle at a temperature of from about 110.degree. C. to about 200.degree. C. to a higher inherent viscosity of from about 0.60 to about 2.50, the higher inherent viscosity being at least 0.10 higher than the inherent viscosity reached in the first step.
The elastomeric polyetheresters so produced have utility as a molding, extruding and sheet forming material, useful for example in the production of plastic tubes, bags, etc., where good physical properties are required.